I think only two could cross-over - the more 'real-life' stories of 'Bouncer' and 'Biker' - the others are all too different to each other to mix. The Wolf series characters don't believe in vampires, so they can't cross with Children of Sol. Finley Moran is a kid, but wouldn't gel with Dusty (teen). Daughters of Le Fey is one on its own and there's no wiggle room to merge and Celtica... I suppose she could actually merge with vampires, werewolves, half-demon teens and possibly even Finley's world.